Hard data structure and algorithm practice puzzle.
Given a list where every value appears three times except one, return the one that appears once. XOR doesn't work here, because it only cancels pairs. Instead, for each of the 32 bit positions, count how many numbers have a 1 there. If the count isn't a multiple of 3, that bit belongs to the answer.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Reading Flag Store, a pipeline you can run and download. See every Software Engineer project.
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